The semiconductor industry currently uses different types of semiconductor-based imagers, such as charge coupled devices (CCDs), CMOS active pixel sensors (APS), photodiode arrays, charge injection devices and hybrid focal plane arrays, among others, that use an array of micro-lenses. Semiconductor-based displays using micro-lenses are also being developed.
Micro-lenses may be formed through either a subtractive or an additive process. In the additive process, a lens material is formed on a substrate and subsequently is formed into a micro-lens shape.
In conventional additive micro-lens fabrication, an intermediate material is deposited in an array onto a substrate and formed into a micro-lens array using a reflow process. Each micro-lens is formed with a minimum distance, typically no less than 0.3 microns, between adjacent micro-lenses. Any closer than 0.3 micrometers may cause two neighboring micro-lenses to bridge during reflow. In the known process, each micro-lens is patterned as a single square with gaps around it. During the reflowing of the patterned square micro-lenses, they form a gel drop in a partially spherical shape driven by the force equilibrium of surface tension and gravity. The micro-lenses then harden in this shape. If the gap between two adjacent gel drops is too narrow, they may touch and merge, or bridge, into one larger drop. The effect of bridging is that it changes the shape of the lenses, which leads to a change in focal length, or more precisely the energy distribution in the focal range. A change in the energy distribution in the focal range leads to a loss in quantum efficiency of and enhanced cross-talk between pixels. The gaps, however, allow unfocused photons through the empty spaces in the micro-lens array, leading to increased cross-talk between respective photosensors of adjacent pixel cells.
It is desired to form a micro-lens array having differently sized micro-lenses. If the known techniques, which use a single reflow step, were used to form such micro-lenses, the differently sized micro-lenses would have different focal lengths, which would lead to poor focusing at certain photosensors or to the need to modify the locations of some photosensors.
It is desirable to enhance the amount of light received from the micro-lenses to the photosensors of an imager. It is also desirable to form a micro-lens array with varied sized micro-lenses, each having a focal length optimized for the color or wavelength of light it is detecting.